The present invention relates to ultrasonic image reconstruction tomography and more particularly transverse tomography.
Transverse reconstruction tomography is a technique of obtaining the image of a cross-section of a specimen object in a plane perpendicular to an axis of this object; it comprises the steps of measuring and recording the action of the specimen on a radiation for a large number of radiation paths contained in the cross-section plane and of determining, by means of an algorithm or similar tomographic decoding process, the action of each individual minute zone of the specimen in the cross-section.
Transverse axial tomography has been in use for a long time in medical X imagery. A description thereof may be found in J. AMBROSE, British Journal of Radiology, 46, pp. 1023-1047 (1973). It has also been used in nuclear imagery (see O. E. KUHL and R. S. EDWARDS in "Radiology", 80, pp. 653-662, 1963). Finally, the application of transverse reconstruction tomography to ultrasonic imagery has been recently contemplated (J. F. GREENLEAF et al, "Acoustical Holography", Vol. 6, Plenum Press, New-York, 1975, pp. 71-90).
Tomography of a living organism by ultrasounds is much more attractive that by ionizing radiations, such as X rays and gamma radiation emitted by radio-elements. Moreover, it is possible to carry out not only measures of absorption, but also measures of time of flight, whereby two pieces of information are collected instead of a single one.
On the other hand, most prior art processes of ultrasonic reconstruction tomography have serious shortcomings. The transmitting and receiving probes are moved between two successive measurements or between two successive series of measurements, which leads to a complex and costly apparatus, slow in operation. However, in another prior art technique (U.S. Pat. No. 4,074,564 to ANDERSON), a plurality of fixed transducers are placed in spaced positions about the periphery of the specimen and each transducer in turn may be energized to transmit a burst of ultrasonic energy which is detected by the other transducers. While that technique improves upon the use of movable transducers, it has a low signal/noise ratio and lack of homogeneity of the transmitted beam since the beam should be fan-like with a very large angular aperture.
It is an object of the invention to provide an ultrasonic reconstruction tomography apparatus which does not include moving parts and delivers an improved image of the cross-section of a body.
It is another object of the invention to provide a reconstruction apparatus which achieves electronic scanning as well as electronic transmission and/or detection focusing.
These and other objects of the invention are obtained by carrying out several successive measurements of attenuation, i.e. absorption, time of flight (which is representative of the velocity of sound through the object) or Doppler shift along several paths in the sectional plane. The paths are directed along a plurality of different angular directions to cover substantially the whole cross-section. Detection is made with a plurality of spaced elementary receiving probes some of them only are used for each measurement.
To this end, the invention provides a process for ultrasonic transverse reconstruction tomography. Switching over from one measurement to another is achieved by electronic scanning and without moving the probes. The signals supplied by the elementary receiving probes are stored and the distribution of the absorption coefficients, of the flight speeds and Doppler frequency shifts across the section, is reconstituted by processing the stored data. The ultrasonic beams are focussed by associating several adjacent transmitting probes which cooperate in delivering the same energy burst.
There is also provided an apparatus which comprises a crown of ultrasonic probes spaced angularly apart about an axis intended to be placed perpendicularly to the sectional plane of the object and means for applying to the probes or groups of successive probes energization pulses, so as to create successive ultrasonic beams directed along several different directions in the sectional plane without mechanical movement of the crown, means for collecting the signals received in response to the transmissions by probes or groups of probes, and means for storing the signals provided by the receiving probes, with a view to reconstituting the distribution of absorption coefficients or flight speeds by processing the signals memorized.
Electronic scanning removes the problems associated with movement of the probes. Slow scanning may be carried out, by providing only one measuring circuit which is connected successively to the different receiving probes, or more or less rapid scanning by providing a smaller or greater number of simultaneous measuring channels.
The apparatus also allows concentration of energy at transmission or reception to be achieved by a process which may be qualified as electronic "focusing" or deflection. For transmission of a single "focussed" burst, a group of probes are energized with time or phase lag corresponding to preferential transmission of energy in a given direction. That may be obtained, when probes are energized by a single electric pulse, by means of delay lines. At reception, a comparable technique may be used, using phase-shifters or mixers having a phase signal different from one mixer to the next.
A particularly important field of use of the invention is for ultrasonic medical imaging. It allows an image to be obtained of a slice of tissue whenever there is involved an organ with a relatively homogeneous composition, which is the case particularly with the brain, breast and neck. It is also applicable to limbs since the high velocity zones are centrally located.
The invention will be better understood from the following description of particular embodiments, given by way of examples only.